In my previous post I discussed when deleting rows from parent tables, how Bitmap Indexes based on the FK constraint can prevent the expensive Full Tables Scans (FTS) on the child tables but not the associated exclusive table locks.

With only a visible Bitmap Index in place, does that mean we’ll have table locking issues if we delete a parent row with current transactions in place ? Let’s check it out.

In one session, we have a current transaction on the child table:

SQL> insert into bowie_kid values (1000001, 'LOW', 4);
1 row created.

In another session, we attempt to delete a parent row (with an ID = 1 which doesn’t currently exist with the child table):

SQL> delete bowie_dad where id = 1;
1 row deleted.

We note the DML was successful and didn’t hang. This means the B-Tree index is clearly being used to police this constraint, even though it’s currently invisible.

In a third session, we now attempt to insert a child row using a FK value that’s in the process of being deleted:

SQL> insert into bowie_kid values (1000003, 'HEROES', 1);

As expected, it hangs as it’s currently effectively waiting on the row level lock made possible by the index entry in the B-Tree index as invisible indexes are still maintained behind the scenes. If we look at a fresh block dump of both indexes, beginning with the Bitmap Index:

It has been updated and lists a new index entry C1 02 as the minimum value now in the index.

So the B-Tree index can be used to successfully police the FK index and prevent the possible table level locking issues associated with deleting parent rows, even though it’s invisible and there is an equivalent visible Bitmap index in place. Invisible indexes are simply not considered as viable execution paths by the Cost Based Optimizer, but may still be “visible” in a variety of scenarios such as quietly policing constraints behind the scenes.

Do I recommend creating two such indexes in Oracle Database 12c. Well, no as the costs of maintaining both indexes need to be considered. But I certainly do caution simply making indexes invisible and expecting the database to behave in exactly the same manner if the index were to be subsequently dropped.

Because rolling back all the above and then dropping the invisible index: